The Roman Moo-Stery is the third book in the Cows In Action series in which the Professor Angus Mcmoo, Pat and Bo travel to the C.I.A (Cows In Action) headquarters to find a Ter-moo-nator attacking! Yak tells them that a Ter-moo-nator is stealing electronic equipment and taking them back in time. So Mcmoo, Pat and Bo chase the Ter-Moonator back in time to the Roman Empire. Bo charges out of the Time Shed without putting her Ringblender on. She is soon caught by the sneaky Lanista (A white water buffalo who is working for the F.B.I).
Mcmoo and Pat encounter Bessium Barmer, one of Bessie Barmer's relatives who complains about the Circus Maximus being closed for 'improvements'. Then a fire starts up and Pat and Mcmoo put it out but see Ter-Moo-Nator's eyes through a hole in the floor. Mcmoo sends Pat out to find Bo and is then offered dinner with the infamous Roman emperor Nero. However Pat is ambushed by the Ter-Moo-nator and is captured.
Meanwhile Lanista is teaching local cows (And Bo) knowledge and toughness. He tells Bo that she shall join the Elite. Elsewhere Mcmoo is having dinner with Nero. Mcmoo warns Nero about evil creatures being at work in Rome but he doesn't listen. Then they both leave to host a special event at the Circus Maximus to cheer everyone up. But Mcmoo realises that Nero's other friend is the Ter-Moo-Nator (who is named T-117), cleverly disguised by his own Ringblender. He then discovers that Nero is working for T-117. Then Pat is thrown to the lions and he offers to help the Ter-Moo-Nator if they let Pat go.
But as Pat is led away from the arena his guards are attacked by Bo and fall unconcious. She explains about the classes to Pat and reveals that Nero is the man behind all of it. But they are forced to hide and be quiet as the ther Ter-Moo-Nator clanks past. Mcmoo is led into a workshop underground and is shown the F.B.I's secret weapon, the War Wagon! He is then forced to fit them with Mega-Thrusters. T-117 explains their plot to overthrow the Roman Eand the world using future weapons. Pat escapes.
Lanista leaves to give Bessium a scroll but Pat changes it at the last second but is then captured again. He to is brought to the workshop. But up above Bo is practicing driving a War Wagon when she reverses into the workshop to let Mcmoo and Pat escape. They enter her War Wagon and are chased into the arena. Lanista orders the War Wagons to fire their weapons at where the most important people of Rome are meant to be without realising that he is destroying his own army. He then enters the arena but Mcmoo self destructs his Mega-Thruster.
The F.B.I agents then aborted their mission and were captured by Yak.