Cratermass is first seen when Amy Rose and Tekno the Canary find themselves in a spaceship, lost, and heading for Earth. Cratermass has waiting for the appearance of aliens for years and is keen to meet these visitors. The initial encounter is awkward, with Cratermass communicating in slow, broken English, despite the girls being fluent in his language. After showing Amy what real hedgehogs look like and that they "might" be more newsworthy than Oasis, the professor reveals that he has been creating a gateway to "infra-space", allowing him access to any planet in the universe, but he can't finish his work. Tekno steps in, but the three of them are captured by Colonel Granite.
Locked up, Cratermass reinforces Amy and Tekno's story that they're just trying to get home, not launching an invasion force. Later, when they've escaped, the professor leads them to their stashed spaceship, himself hanging onto the side as they fly home. Back at Cratermass' house, he and Tekno work together to complete the gateway, as Granite's forces approach. Soon enough, the portal appears and Amy and Tekno beam home. As Granite appears, Cratermass deletes the program with one touch of a button, effectively ending Granite's ambition of capturing the Mobians.
It is not known what happened to Cratermass, but it is likely he wasn't treated kindly by Granite.
- Cratermass' name is based on the popular British sci-fi series, "The Quatermass Xperiment". Unlike Cratermass who is all for a visit from interplantary beings, Quatermass spends his series preventing alien invasions.