Frankie and Benjy (also called Frankie Mouse and Benjy Mouse) were two white mice who belonged to Trillian as her pets, which she had taken with her from Earth. It is later revealed that they are actually pan-dimentional beings who were behind the ten-million-year research program to find the Ultimate Question; the research program being the Earth itself.
Frankie and Benjy, while on Earth, were two of the most intelligent beings on the planet, as mice were the most intelligent species on Earth. They later travelled on the starship Heart of Gold with Trillian, Zaphod Beeblebrox and Marvin, later joined by Arthur Dent and Ford Prefect.
While in the Heart of Gold, above the surface of the planet Magrathea, the mice escaped. Trillian was greatly unhappy with the disappearance of her mice, however the other members of the crew were not as bothered, especially Zaphod, who said: "Oh, nuts to your white mice." Later, when Trillian reminded him about the search for her two pet mice, Zaphod responded by saying ". . . And two white mice if you insist" and "Oh your bloody mice!"
The two mice escaped the ship and made it onto the surface of Magrathea, with Trillian finding their mouse droppings while travelling down a cave within the planet.
After Arthur has been told by Slartibartfast about the history of Magrathea, and of the mice and their experiments with Earth, he rejoins the others in a dining room on the planet. Zaphod explains to Arthur that their hosts - the two mice - attacked them with a "fantastic" dismodulating antiphase stun-ray, then invited them to a meal, to make up for attempting to harm them.
Benjy welcomes Arthur, and then both him and Frankie are introduced by Trillian. It is unknown if these are their real names which they told the others, of if these are the names given to them by Trillian, which they like to be addressed by. The mice dismiss Slartibartfast then explain their issues with obtaining the Ultimate Question. Frankie tells them that the two of them are "sick to the teeth" of the whole situation, and that the idea of having to re-do their experiment gives him "the screaming heebie-jeebies".
Frankie and Benjy have been invited to do chat shows and lectures, however they feel that having the Ultimate Question would give them even more bargaining power with the Five-D television companies.
In the radio series, Frankie and Benjy offer Arthur and Trillian a large amount of money in return for finding information on the Ultimate Question. Zaphod seems keen on the idea, and even bargains for more money, however the police arrive on the planet in search of Zaphod and the stolen Heart of Gold ship, and so a deal is never finalised. Benjy mouse makes a final attempt to implore Arthur to search for the Ultimate Question, and Zaphod says that they will find it, with no real conviction, before leaving the mice.
In the book and television series, the two mice claim that they only way for them to get the Ultimate Question is for them to remove Arthur's brain, with the police again interrupting before the mice can take any real action. In the film, Frankie and Benjy are revealed to be Lunkwill and Fook, who were part of the pan-dimensional hyper-intelligent race that created. They are killed by Arthur when they attempt to remove his brain, resulting in them fading out as they permanently vanish.