You might try plausible. From NOAD:

> **plausible** |ˈplôzəbəl|
*adjective*
(of an argument or statement) seeming reasonable or probable : *a plausible explanation* | *it seems plausible that one of two things may happen*. See note at **believable**.

Or look at the usage notes under *believable* for a synonym:

> THE RIGHT WORD

> **Believable** is the most general of these terms, used to describe anything we accept as true, even in the absence of absolute proof (: *a believable story about why she was late*).

> **Credible** also means worthy of belief or confidence and is often used interchangeably with believable, but it goes one step further: a credible excuse is one that is supported by known facts.

> **Creditable**, often confused with credible, at one time meant worthy of belief but nowadays is used to mean respectable or decent, deserving of honor, reputation, or esteem (: *leading a creditable life*).

> Something that is **convincing** is believable because it overcomes doubts or opposition (: a convincing performance), while something that is **plausible** may appear to be convincing or believable on the surface, but may not be so upon closer examination.

> **Valid** means legally sound, just, or authoritative; *a valid criticism seldom provokes opposition*.

> **Cogent**, on the other hand, means having the power to convince; a cogent argument is believable because of its clear, forceful, or incisive presentation.