Depending on how involved the deities are in your campaign...
Perhaps each one of the group has received a particular religious vision (or a dream) from their patron deity (or deities) warning them of a great evil rising--perhaps a vision of the LotR: RotK scene you mentioned. Perhaps the PCs must go on a quest to find a means to stop this evil before it's too late, or they must try to muster the sleeping might of the "good" nations before this tide of evil sweeps over and drowns them all.
Then again, there's always the "childhood friend" angle--esp. between the rogue & 1 of the other PCs (in order to explain the rogue's presence). They don't literally have to have been childhood friends--at the least, they could have been friends for a few years, before either of them "technically" became a 1st-level character. Seems like the human & halfling could form this bond easily, but anything goes.
Sort of a spin on the 1st idea--perhaps the rogue (only) received the aforementioned foreboding dream/religious vision (from a prominent Good &/or Lawful deity, or one of the deity's known sevants, like a celestial or saint). So far, the rogue has gained only 2 allies who accept the veracity of her dream/vision--a paladin & a monk (either from the same Lawfully-aligned faith or not).
Then again, the rogue doesn't necessarily need to be a "thief," per se. Perhaps the rogue is more of a scout for a local army, the paladin is an "elite" city guard or soldier, and the monk could be a bodyguard, local scholar, or other official representative (perhaps a diplomat or even a non-magical field medic, if you will).
Hope this helps.