A21 was a prose fiction series published on the Gerry Anderson website, heavily based upon TV Century 21 and Action 21, and featuring elements introduced in those magazines.
- You may be looking for its in-universe counterpart or Action 21.
Crossovers
As the series featured elements from TV Century 21, it had some minor crossovers with elements of the Doctor Who universe, such as Astra and Unity City.
The following A21 stories and features involved DWU elements and are thus covered on this Wiki: