The ClueFinders 6th Grade Adventures: Empire of the Plant People is a computer game released in July 19, 1999. In it, Owen and Leslie must investigate a secret underground civilization of intelligent plant life in order to rescue Joni and Santiago.
The game begins with the ClueFinders playing with a frisbee in Joni's backyard. The ClueFinders are greeted by Joni's elderly neighbor, Ms. Rose, who tells them that she's busy taking good care of her plants. When the ClueFinders return to their game, Joni throws the frisbee too far and it goes into Ms. Rose's yard. Joni and Santiago go to look for the frisbee, but just as they locate it, they are swallowed up by a hole in the ground, which closes after they fall in. Owen and Leslie hear Joni and Santiago yell, but when they go to look, all they see is the frisbee. They establish communication with Joni and Santiago using the video phone, and Joni explains that they appear to be in some kind of underground chamber. Owen, Leslie, and LapTrap set out to find and rescue Joni and Santiago.
After getting through a locked door in Ms. Rose's yard, Leslie and Owen suddenly find themselves in an underground plant kingdom. Some plant guards notice them and try to capture them. As Owen and Leslie try to escape, they are suddenly pulled aside by a small plant creature. He tells them he's on their side and introduces himself as Ficus. He explains that the underground plant kingdom has existed for centuries without humans knowing about it. Lately, something has gone wrong, and the plant soldiers have begun taking humans captive and are planning to attack the town above. Ficus says that he and his friends are against this, but they can't convince the soldiers to stop, so Ficus turns to Owen, Leslie, and LapTrap for help. When Leslie asks Ficus about the whereabouts of Joni and Santiago, Ficus explains that the leader of the plant kingdom would know, and in order to get to the throne room without getting caught, they will need to build bridges out of petrified wood planks to get across the bottomless pits.
While exploring the underground plant empire, Owen and Leslie notice that something is very odd about the water. It seems to have become discolored, and they come to the conclusion that it contains industrial waste. They also find Ms. Rose's walking stick, which leads them to the conclusion that she was captured by the plants too.
After Leslie and Owen finally reach the throne room, they are suddenly ambushed by plants and knocked out. When they come to, Ms. Rose has appeared before them. She then transforms into a large, monstrous plant and reveals that she was the leader of the plant kingdom all along. Ms. Rose explains that the kingdom's residents had lived peacefully for centuries until pollution from industrial waste poisoned the kingdom and made its citizens angry and aggressive. They plan to attack the town above in retaliation because they feel it is all they can do to save themselves. The ClueFinders try to convince her that she shouldn't attack the town and that they can stop the pollution themselves. Ms. Rose agrees to give them one last chance, but only on the condition that she can keep one of them hostage for security. Joni agrees to stay behind while Santiago goes up to the surface to report the contamination to other people, and Leslie and Owen work with the plant kingdom's scientists to find a way to purify the water.
After making some progress, Owen and Leslie report back to Ms. Rose, and they are alarmed to see that Joni has started transforming into one of the plant people. Ms. Rose tells them that they had to keep their captives fed, but everything they could give them was polluted, and so the captives started transforming. Ms. Rose tells them that only clean water can save the captives and that they must hurry. After Leslie and Owen find a way to purify the water, the plant kingdom becomes healthy again, and Ms. Rose is very happy and grateful to the ClueFinders for their work. She returns Joni to them, who has reverted back to normal and is sleeping. Ms. Rose explains that all the captives have fallen asleep, and when they wake up, they won't remember the plant kingdom at all. Ms. Rose asks that the ClueFinders keep the plant kingdom's secret, which Owen and Leslie swear to. Ms. Rose then bids farewell to them as they depart the kingdom.
- Mysterious Locks
- Skills: Reading comprehension, world cultures, geography, biology
- Steps of Expression
- Skills: Activity focus: vocabulary, grammar, reading comprehension
- Perilous Faults
- Skills: Spelling, vocabulary
- Recipe for Disaster
- Skills: Math computation, decimals, fractions
- Cave of Danger
- Skills: Math computation, decimals, fractions, estimation
- Chasm of Time
- Skills: History, world cultures
- Pollution Detectives
- Skills: Reading for information
- Bacterial Growth
- Skills: Logical reasoning with word problems
- Pollution Solution
- Skills: Logical reasoning
- This game reuses several music tracks from The ClueFinders 5th Grade Adventures: Secret of the Living Volcano.
- One of the tracks in this game sounds similar to Enya's "Cursum Perficio"
- The box art depicts Joni wearing the backpack like she does in most other games, despite that Leslie is the one who wears it in the game and due to having Owen and Leslie themselves play the primary roles whilst Joni and Santiago played secondary roles.