I, Zombie is a very strategic puzzle game. First, you 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, you must have a good understanding of the abilities of both plants and zombies. For example, if a row has only two or three cardboard Peashooters, it is safe to say that planting a Buckethead Zombie or a Football Zombie will earn you a brain and further them to completing the level. You cannot take back zombies they placed, so you must plan wisely according to their amount of sun left. Each Sunflower eaten will give 200 sun to the player.
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 that the player can only get sun from getting rid of Sunflowers and for the mushrooms to stay awake.
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 Sunflower is the only plant that appears in every I, Zombie level.
This is because it is needed to produce sun to place the Zombies on these levels.
"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.
Every time a Gargantuar smashes a plant or a brain, a single eating sound is heard.
I, Zombie is the only game in Puzzle Mode that does not return in Plants vs. Zombies 2 (excluding Chinese versions of the game)
Gargantuars can still throw their Imps in I, Zombie, but this does not usually happen; as the 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 mower.
I, Zombie is not available in the Nintendo DSiWare version of Plants vs. Zombies.
Doing an I, Zombie level without mushrooms (like the first one) will get you the No Fungus Among Us achievement, except in the iOS version of Plants vs. Zombies.
On the Nintendo DS version of Plants vs. Zombies, the plants do not 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 do not eat the brain of the homeowner after reaching the house. However, Crazy Dave states that he put the cardboard plants in front of the player's house on the first I, Zombie level.
In the iPhone and iPod versions, the first level is free but the other eight and endless are purchased through Crazy Dave's Twiddydinkies 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, 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. This does not work on the iPhone version, as only one Bungee Zombie can target a space at any given time.
In the Nintendo DS version of Plants vs. Zombies, most zombie's seed packets only shows the head, except for the Imp's seed packet, as its 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 of Plants vs. Zombies, 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 of Plants vs. Zombies, after nearly reaching the player's house, the zombies suddenly disappear.