Teenage Mutant Ninja Turtles: Shredder's Revenge has followed the Streets of Rage 4 playbook to the letter. Resurrect a beloved beat-'em-up franchise? Check. Create arguably the definitive entry in that franchise? Check. One year after launch, release paid DLC with extra playable characters, new music, and a rogue-lite survival mode? Check mate. More TMNT is never a bad thing, of course — especially for Gen Xers and Millennials who grew up with the IP — but is this new expansion, Dimension Shellshock, as indispensable as the base game?

The major selling point of Dimension Shellshock is the new "Survival" mode, which takes the Ninja Turtles and their allies across parallel dimensions in an attempt to halt Shredder once and for all. It all starts with the Neutrinos, the friendly aliens from Dimension X, who interrupt a pizza party to announce the dreadful news that Shredder plans to conquer the multiverse. Soon, the terrapin teenagers and their friends are off into the unknown to save the day.

The rules of Survival mode are fairly straightforward. As you jump through time and space, you'll collect crystal shards by defeating enemies. Once you collect the required number of shards, you'll complete a Dimensional Crystal, which slingshots you into the next dimension and ups the difficulty. If you're lucky enough to collect five Crystals, you'll earn the right to fight the master of the void. Even if you fail during your run, your amassed Crystals will convert to new abilities and power-ups, for example +1 hit point or +1 life — not to mention special skins. Thus, the mode embraces a rogue-lite flow whereby early failed runs translate to a more capable fighter, and thus a greater chance for future success.

Survival mode adds something important to Shredder's Revenge. It makes the package more challenging and more replayable. For all its greatness, the original game, like most beat-'em-ups, suffered from a short running time; you could see the credits roll in one sitting. This new mode helps to fix that, with its endless gameplay, online leaderboards, and upgrade pathways for all nine playable characters. It's certainly the most addictive element of the game.

It's also, unsurprisingly, the most repetitive. Due to its roguish gameplay loop and live-die-repeat cycle, it will invariably present many of the same backdrops, enemies, and scenarios. Because of this — just like the DLC for Streets of Rage 4 — it cannot match the tailor-made excellence of the campaign in Shredder's Revenge. Developer Tribute Games certainly tries, though. Not only does the studio introduce five different dimensions, each with a unique look and feel, not to mention tons of fan service, but also several different random modifiers and perks to keep the action fresh. 

Modifiers are special events that include low gravity, gale force winds, faster enemies, and exploding enemies, among others. Perks, chosen by the player after clearing each wave, cover everything from "Rude Dude", which doubles both attack power and incoming damage, to "Sushi Chef", which conjures a plate of sushi for every 10 successive combo hits.

While perks help reduce tedium, they arrive with their own issues. For one, it's unclear what exactly a perk does during early runs before you've committed them all to memory; for another, many of the perks simply aren't that helpful. Finally, and perhaps most troublesome, the perks don't stack for an entire run, thereby removing one of the most attractive elements of a rogue-lite: mixing and matching buffs to give yourself a greater chance for victory the deeper you go. In general, you're not amassing assets and sacrificing others to create an ideal build in Dimension Shellshock; you're mostly just taking it one room at a time. The inclusion of more valuable and more varied perks, which complement each other and last for the entirety of a run, would allow Survival mode to hit that next tier.

Although the majority of perks won't make a huge difference, there is one major exception: the ability to assume the form of a boss bad guy like Bebop, Rocksteady, or even Shredder. Wiping out waves of foot soldiers with Rocksteady's blaster or Shredder's jump kick is pure power fantasy. It never gets old.

Outside of Survival mode, Dimension Shellshock introduces two new playable fighters, Usagi Yojimbo and Karai. Both are welcome additions to the already large roster. Usagi is a determined rabbit samurai with high speed statistics and whose launchers & mid-air attacks make him an expert enemy juggler. Karai, a Foot Clan lieutenant, is even better — which pains me to say, since Usagi was my favorite TMNT action figure as a kid. In fact, Karai is immediately the coolest, most exciting character in Shredder's Revenge. Her moveset is literally electric. She bounces around each arena with lightning-fast combos and devastating super moves. Even her throws are badass. She grabs enemies by the head and tosses them aside or leaps off the screen and then piledrives them into oblivion.

The last piece of the Dimension Shellshock puzzle is a new OST from Tee Lopes, who scored the base game. It's another solid, energizing soundtrack from Lopes. Highlights include the all-or-nothing "Dire Horizon", the rocking "Four Red Masks", and the NES-inspired chiptune anthem "I 8 a Bit much".

Like the Streets of Rage 4 expansion Mr. X Nightmare, Dimension Shellshock isn't great on its own, but it does make an already great game even better. If you've been looking for a reason to boot up Shredder's Revenge, this is it. Survival mode adds significant replay value, even as it suffers from repetition and unhelpful perks, and the new playable fighters, particularly Karai, are strong additions. Plus, there are slick new tunes from fan-favorite Tee Lopes. It's time, once again, to kick some shell.