Here are some synonyms for "implausible":
* Unlikely
* Unconvincing
* Far-fetched
* Incredible
* Absurd
* Outlandish
Example:
"The detective's theory about the murder was implausible, as it relied on a series of improbable coincidences."
In this example, the detective's theory is described as "implausible" because it is not believable due to the unlikely events it relies on.