The spies must protect a ship carrying the last component required to make a nuclear bomb. They use Frank's mini-submarine to follow the ship and are making good progress underwater when the ship is attacked.

An archaeological dig has come to St Hope's to investigate the mysterious disappearance of a Roman legion, and it is getting worryingly close to HQ. The spies must stop the archaeologists from uncovering HQ.

Mr Flatley receives a letter from his Great Aunt Jemima informing him that she has left part of her fortune to him in her will, but also receives a ransom letter threatening to blow up the school unless he turns the money down.

Olaf and Hannah Stein are the cleverest people in the world. They have developed the MI9 computer firewall and no one else in the world can crack it, so when they are kidnapped, the spies are left holding their baby, Grace.

Terry Zucker has invented the best vacuum cleaner in the world - just turn it on and it cleans up all by itself. Every household in the UK has one, including the prime minister. Meanwhile, an important statue has been stolen. The team uncover the culprit, but in the struggle to capture him they accidentally switch on a vacuum capable of creating a black hole.

The spies receive some intel about filmmaker Lars Von Tripod - every time he makes a film a valuable artefact is stolen. The team lure Von Tripod to St Hope's with a script that features three school pupils who are also spies.

The spies break into MI9 and steal important information, only it's not really them, but robots that look identical to Rose, Carrie and Oscar. Stark is in pursuit of the spies, convinced that they are the culprits. They must prove their innocence and release Frank from an MI9 torture chamber.

The Head of M.I.9 and Stark summon the team to their office. Something big has happened. Jade Dixon Halliday has sent an urgent plea for help. Her cover has been blown within SKUL and she needs to be broken out.

Financial codes from the Bank of England are being leaked and the spies determine that a restaurant owned by celebrity chef Tommy Blumenheck could hold the answer.

Mrs King's memory wipe is failing. Soon she will remember that Rose, Carrie, Oscar and Frank work as spies for MI9. Frank and the spies develop a strategy to help Mrs King forget once and for all. They construct an elaborate mission for her to complete as a real spy. If it works, she should think it was her subconscious mind playing tricks on her and it was all just a dream.

The UK is hit by a massive earthquake, with multiple epicentres all over the country. Professor Quakermass, a brilliant seismologist, contacts Frank. He thinks that there is more to these earthquakes than meets the eye.

A crime wave is sweeping the nation and SKUL are responsible - or are they? Not content with being an underling, the Grandmistress has conspired to get rid of the Grandmaster and take over SKUL. The spies must stop her fluffy, animatronic army and put an end to the Grandmistress' reign once and for all - but it seems an impossible mission. Will they succeed, whilst keeping their jobs and integrity?

The Grandmaster sets off a Despair Bomb that emits fear, anger and hate, leaving the entire country grey and miserable. Carrie's optimism is the only thing that can stop the effects of the bomb and save the world.