Finding theoretical mechanism of sodium thiosulphate and hydrochloric acid

In class we conducted an experiment where we timed reactions between sodium thiosulphate and hydrochloric acid. We kept the concentration of the $\ce{HCl}$ constant, while changing the concentrations of sodium thiosulphate. 


We then graphed this data to discover there is a linear relationship between the concentration of thiosulphate and 1/time, indicating the it is a first order reaction. This means in the rate determining step there will be one thiosulphate ion.


In the analysis question it asks you to come up with a theoretical mechanism. I'm having alot of trouble with this because the first order relationship found on the graph indicates there is 1 thiosulphate ion in the rate determining step, but the overall equation also has one thiosulphate ion.


Can anyone help me out, I have tried multiple methods but none seem to add up to the overall equation.


$$\ce{2 HCl (aq) + Na2S2O3 (aq) -> 2 NaCl (aq) + SO2 (g) + S (s) + H2O (l)}$$


