I, Zombie is a group of Puzzle Mode levels in which the player must "plant" zombies (they must be planted behind the red line, except for Bungee Zombies) to overcome cardboard plants and get to the brains at the end of each row. The level is over when all of the brains have been eaten. At the beginning of the first level, Crazy Dave tells the player that the zombies want to practice invading houses, and he did not want any plants harmed, so he put cardboard plants on the player's lawn. The last level, I, Zombie Endless, never ends. After every three streaks of I, Zombie Endless, a present containing a Zen Garden plant, chocolate, or a money bag with gold coins or a diamond usually drops, although it will not always happen.
I, Zombie is a very strategic puzzle game. First, the player must find ways of getting the brains whether by using Bungee Zombies to take Sunflowers, Gargantuars to smash them, or other zombies to eat them. Also, the player must have good understanding of the abilities of both plants and zombies. For example, if a row has only a few cardboard Peashooters, it is safe to say that planting a Buckethead Zombie or a Football Zombie will earn the player a brain, and further them to completing the level. The player cannot take back zombies they placed, so they must plan wisely according to their amount of sun. Note, however, that each Sunflower will give 200 sun when eaten.
There are ten levels of I, Zombie, which get harder in succession. All of the levels take place in the equivalent of the Night stage of the game, presumably so the player can only get sun from getting rid of Sunflowers and for the mushrooms to stay awake.
"I, Zombie" is a reference to the book and film I, Robot.
"Three Hit Wonder" is a reference to the term "one hit wonder" (the number three comes from the number of hits the Imp can take). If an Imp is placed right next to the line in a row with only a one normal damage shot attacking plant (such as a Peashooter, Puff-shroom, Fume-shroom, etc.) at the front, the Imp will have approximately a forty percent chance of eating the attacking plant and may even eat the brain.
In the level Me Smash!, Gargantuars smash the brains (and a squashed Brain image appears on the ground) instead of eating them. This is because Gargantuar has no eating animation.
Gargantuars can still throw their Imps in I, Zombie, but this doesn't usually happen; as Gargantuars are only capable of throwing their Imps if they are less than halfway across the lawn and the line marking where zombies can be placed. Unless there is Squash next to the line, in which case if the closest plant to the brain in the same lane are Sunflowers or Spikeweed, the Imp may eat the Brain.
The code "dance" makes the Zombies move somewhat faster than usual, so typing dance until you see the Zombies go at their speed limit is a partial advantage.
It is unknown whose Brains are there in place of the Lawn Mowers. These brains may have been also cardboard cut outs made alive by the Magic Taco; it is also possible that they are animalbrains, not human brains or cardboard.
I, Zombie isn't available in the Android and DSiWare version.
Doing an I, Zombie level without mushrooms (like the first one) will get you the No Fungus Among Us achievement and is the easiest way, except in the iPad version, iPod Touch, and iPhone.
On the Nintendo DS version, the plants don't look like cardboard and if the player uses Action Replay to give plants rapid fire, the plants are affected.
It is unknown why the Zombies don't eat the brain of the homeowner after reaching the house. This might be because it is a neighbor's house. However, Crazy Dave states that he put the cardboard plants in front of the player's house on the first I, Zombie level. One possibility is that Crazy Dave instead put the cardboard plants in front of a different house that does not have anyone living in it, or the zombies are already full by eating the other brains. It can also be that the house is locked so the Zombies don't come in, but then it is unknown why the homeowner didn't do this during Adventure Mode, although this may be because the zombies will ultimately enter. It can also be that the zombies knew this was only practice and they wanted to wait until the real attack to eat the homeowner's brain. However, the zombies are not very smart, hence their desire to eat brains. The homeowner may possibly be out of the house, planting zombies, but that will give a Digger Zombie chance to eat the homeowner's brain. Since most of the I, Zombie levels are only available after completing the Adventure Mode(and a party occurs just after, putting a truce between the neighborhood and the zombies), it may mean the reason why no human brain gets eaten is because the truce still holds; however, three levels are available before completing Adventure Mode. It could also be possible that Bungee Zombies grab the zombies that reach the end.
In the iPhone and iPod versions, the first level is free but the other eight and endless are purchased through Crazy Dave's Shop for 150,000 coins.
If the player places a Pole Vaulting Zombie where there is no plants, just a brain (normally where a Digger Zombie has been) it will lose its pole and eat the brain without regaining its pole.
This also happens with Balloon Zombie, the balloon will pop when it reaches the brain and the zombie eats it.
If the player uses Speed hack to slow the game down, or if he or she has very fast fingers in the PC version, he or she can make multiple Bungee Zombies grab the same Sunflower, as for a while after taken, the Sunflower is still on the lawn while the first Bungee Zombie is raised; however, it does not work on the iPhone version; only one Bungee Zombie can target a space at any given time.
In the Nintendo DS version, most zombie's seed packets only shows the head, except for the Imp's seed packet, as it's seed packet shows it's head and part of his body, and the Conehead Zombie's seed packet, which only shows the Roadcone.
In the Nintendo DS version, when choosing where to plant an Imp, the Imp can be seen with a bone.
In the iPhone, iPod, Xbox Live Arcade, PlayStation Network, and PlayStation Vita versions, after nearly reaching the player's house, the Zombies suddenly disappear.