Since strong and broad emission lines are a hallmark of quasar spectra, 
the recent discovery of luminous, high-redshift quasars with strong blue 
continua but extremely weak or undetectable emission lines is puzzling. 
I will present a step-by-step overview of multiwavelength observations 
that provided insights into the nature of these remarkable sources. 
In particular, I will show that the weakness of their emission lines is 
unlikely to be due to line absorption, dust obscuration, gravitational 
lensing or microlensing effects, or relativistic beaming. 
Finally, I will describe an observational and theoretical effort required 
to distinguish between several scenarios that have been proposed to explain 
the weakness of the emission lines in these sources. 
Solving the mystery about this rare class of quasars will shed light on 
emission-line formation and the accretion process in all active galactic 
nuclei.