I think you can read double-density (720k) disks, but not high-density (1.44MB).
EDIT: According to
LowEndMac's specs for that model, the hardware does indeed only support the 720k disks. The Mac Plus will also need System 7.5 as a minimum OS, since that came with the PC-Exchange extension which allowed FAT formatted disks to be read.
EDIT 2: Spoke too soon. System 7 required a hard disk, so unless you have one plugged into the Plus' SCSI port, it's not going to play.