
With the final boss defeated, space colony Nine is safe, and Cynthia's casket is restored.
For the sake of completeness, here's the American Mary's ending, and Edger's ending.
Unlike the arcade game, there's no looping for you- you go right back to the title screen. Good job, Mary!

The evil ALIENS have been defeated, and so Double Target (or Quartet, whatever) has been beaten.
A bit like Laser Ghost, I'm a little torn on this one. On the one hand, it does feel slightly more satisfying than the arcade game in one-player at least, and elements of the co-op-to-counter-op are still present in two-player mode- it's slightly harder to steal the key, but you get medals for killing bosses and there's the fact that you might have to fight over the Jetpack. That said, Double Target is nice enough but only enough for 3 stars. It's sort-of stuck in a weird spot, because it's not really a run-and-gun game like Contra so comparing it to Konami's classic run-and-gun (which saw its arcade release in the same year DT came out) probably isn't sporting, but it's not exactly a traditional platformer either. It has some neat ideas, like finding the hidden Star Powers and missiles/energy, dividing each round into two different sections, and I like the platforming aspects as it feels very weighty and has good jump control, but the main problem is that some of its ideas- the exploratory ones in particular- actually hurt it rather than help.
The fact that enemies respawn constantly and nearly instantly- especially in such cramped conditions like the mazes in Rounds 3 and 5, and the cavern in Round 2- means that to-ing and fro-ing is quite frustrating, especially when it's those UFO enemies or the relentless lips. Respawning enemies in games like Ninja Gaiden (BIRDS) works because they're obstructing your one path- no backtracking is involved, so once you're past an enemy respawn point, that's your lot. Here, though, many of them are a pain in the ass to kill, they respawn constantly which isn't good when you have to go between certain areas- this feeling is exacerbated in areas with little room to manouvre, and it doesn't help that they respawn almost instantly if they hit you. Another thing is the energy meter- it feels like, with some fine-tuning, it could've done fine without it. Although it decreases faster than the arcade game, the number is so much bigger that it really doesn't affect you that much, and to be honest, it feels like the game gives you so much energy because it knows you'll keep bumping into the constantly respawning enemies. Compare this with, well, any other action game of the time with one-hit deaths, which encourage careful play (and pattern recognition, obv.). Double Target really doesn't do that as much.
To be fair, this kind of game was in its infancy at the time, so it's not surprising neither the arcade Quartet or this Master System reimagining get it totally right. It's not that it's a bad game- there are worse games like this, and it does what it does competently enough, and it handles jumping better than the arcade Contra (no goofy somersaults here, the jump is surprisingly controllable)... It's just average, good enough.
Konami just hadn't laid the ground-work for this kind of thing yet, you know?
And now, it's that time, folks!
EXTENDED PLAY!
Cheat Time! Quartet/Double Target has three cheats, all of which are entered on the title screen:
Level Select (2-Player Mode Only)
Press Pause 12 times, then hold upper-left on Controller 2, then press both buttons on Controller 2.
Sound Test
Press Pause 4 times, then press Button 1 on Controller 2.
Start with Wide Beam (1-Player Mode Only)
Press Pause 14 times, then press Button 1 on Controller 1.

Entering the Sound Test or Level Select cheats take you to this screen.
(Mary looks like this even in the US/World version.)
It's some Master System friends! Opa-Opa, Teddy Boy, Alex Kidd and the knight from Pit Pot!
They even move around when you're playing music in the Sound Test.
We'll take this opportunity to mention that our pal Tinpot Gamer reviewed Pit Pot once. Read his site, will you?
Let's look at the differences between Double Target and Quartet! This won't take long.

Obviously, the title is different, and so is Mary.

Her in-game sprite has been changed too, but they forgot to change her life icon- it still has black hair

And here's the results screen. The US Mary is giving me one hell of a glare...
There's also a difference not many people may notice- the layout for Round 1 is changed a fair bit between versions. In Quartet, the first few sets of platforms at the bottom of the screen have been changed so there's less bottomless pits to fall into- the first hole in particular is very easy to fall into in Double Target, but Quartet's layout is considerably safer. You can look at a comparison of the two maps here.

Finally, here's some art from the manuals for both- Double Target on the left, Quartet on the right (scan from ratigators.com).
Note that the US Mary doesn't look like the US Mary at all!
Speaking of... I put together a compilation of all the official art of Mary because every single one is different. It's quite odd.
If you can't be bothered digging out your Master System to get your Double Target on, then where else can you play it? Not the usual place, that's for certain. While Sega did their best supporting the Master System on the Wii's Virtual Console- I certainly wasn't expecting Secret Command to ever return- Double Target was not one of the games made available for it. At least they put Alex Kidd in Shinobi World on there before giving up, eh?


No, for some non-MS Double Target action, you're going to have to dig deeper. Volume 21 of the import-only Sega Ages 2500 collection on the PS2 is S.D.I. & Quartet - Sega System 16 Collection, a double-pack of the arcade versions of S.D.I. (that's Strategic Defence Initiative, ya know) and Quartet, and their respective Master System conversions. Ported by emulation maestros M2, this is a pretty decent set if you happen to be a massive fan of these two games. Otherwise, it's a hard sell. That said, as with all the M2 collections it comes with a good selection of extras- all four games have their own art gallery (including the Quartet flyer manga!), sound test, and for the home conversions, the ability to select any region- especially useful for Double Target/Quartet. Really, it's the die-hard S.D.I. fans that get the most service here- this port is USB mouse-compatible, and beating that game unlocks a production document and sheet music. Quartet fans get no such luck, but the reproduction of the manga in the instruction manual is a nice touch!
On the plus side, the collection's website has some nice wallpapers.
(As a bonus, here's the archived version of the Sega Ages 2500 index in case you needed some Fantasy Zone wallpapers.)
Oddly enough, for a game that Sega seem to have totally forgotten about given the lack of further ports, Double Target is an interesting footnote in Sega history. It was one of the last games Rieko Kodama worked on before she created Master System favourite Phantasy Star. She worked on character design, which probably explains why Mary looks so different in this port... Then again, she looks different in almost every appearance she's ever made. There's an intereview with her on The Sega Collection, and an in-depth overview of her work on Sega-16 if you want to know more, but Quartet/Double Target only gets a passing mention, naturally.
Double Target - Unofficial Soundtrack
Finally, here's the game's soundtrack, ripped from that handy sound test. The first four rounds take their background music from the arcade Quartet- Quartet Theme, Sky, OKI Rap and FM Funk- and while they're not quite as aggressively blue skies as the arcade version, they're pretty decent, albeit with no voice samples, obviously. The rest of the soundtrack is all-new, and the final boss tune in particular is pretty good. So, you know, give this a download, you might like it. That said, one extremely dedicated fan took the Double Target-exclusive tracks and turned them into System-16 songs, and they're really nice that way... It's up to you. I guess it depends on how much you like those goofy voice samples.

The weird thing is, I have this bizarre fixation on the original arcade Quartet. It's so very Sega.
I even own the English flyer for it! How sad is that, eh?
Before we go, though, a reminder to visit GMIX6809's playthrough which was a big help. Well done, super player!
Mary and Edger conquered the evil. They would be legends all around the universe!