Nate the Great and the Mushy Valentine

Nate the Great hates mushy stuff. But when he spies a big red paper heart taped to the outside of Sludge's doghouse, Nate knows he must help out his favorite pooch. Wh has left Sludge a secret valentine?
It's a mystery until Nate finds out his friend Annie is missing a valentine. The case seems easy. Nate is relieved. No more mushy stuff. That's what he thinks.